Automatic transmissions employ the use of hydraulic pressure to engage and disengage clutches and brakes to change gear ratios. Generally, high pressure is maintained on the clutch piston so that the clutch or brake will transmit torque sufficient to propel the vehicle. However, a continuously operating transmission pump must produce high pressure to keep the clutch engaged. Continuous operation of this pump results in decreased fuel economy.
U.S. Pat. No. 4,729,462 employs external levers and actuators to actuate the clutch, resulting in additional cost and space requirements. U.S. Pat. No. 6,771,031 uses electric motors to actuate the clutch, adding the cost of the motor and energy use to maintain engagement.
Thus, there is a long-felt need for an inexpensive and space saving design to maintain torque transmission in a hydraulically applied clutch, while increasing fuel economy by operating a low pressure. There is also a need for a fail-safe device to release a transmission clutch when a minimum pressure cannot be maintained.